RnD project to compare various mixin approaches in TypeScript.

• Getting started
  • Requirements
  • Install
  • Usage
  • Exports
  • Advanced examples
    • applyMixinsAsProxy
    • applyMixinsAsMerge
    • applyMixinsAsSubclass
    • applyMixinsAsProto
    • applyMixinsAsFactory
• Implementation notes
  • Q&A
  • Definition
  • Mixin cases
• Refs
• Alternatives
• License

• Node.js ^12.20.0 || ^14.13.1 || >=16.0.0
• TypeScript >= 3.7 | 4.x

yarn add @qiwi/mixin
npm i @qiwi/mixin

import {applyMixins} from '@qiwi/mixin'

interface IA {
  a: () => string
}
interface IB {
  b: () => string
}
class A implements IA {
  a() { return 'a' }
}
const b: IB = {
  b() { return 'b' }
}

const c = applyMixins({}, A, b)
c.a() // 'a'
c.b() // 'b'

const D = applyMixins(A, b)
const d = new D()

d.a() // 'a'
d.b() // 'b'

The library exposes itself as cjs, esm, umd and ts sources. Follow packages.json test:it:* scripts and integration tests examples if you're having troubles with loading.

import {
  applyMixinsAsProxy,
  applyMixinsAsMerge,
  applyMixinsAsSubclass,
  applyMixinsAsProto,
  applyMixinsAsPipe
} from '@qiwi/mixin'

interface A {
  a(): string
}
interface B extends A {
  b(): string
}
interface C extends B {
  c(): string
}
interface D {
  d(): number
}
const a: A = {
  a() {
    return 'a'
  },
}
const _a: A = {
  a() {
    return '_a'
  },
}
const b = {
  b() {
    return this.a().toUpperCase()
  },
} as B
const c = {
  c() {
    return this.a() + this.b()
  },
} as C

class ACtor implements A {
  a() {
    return 'a'
  }
  static foo() {
    return 'foo'
  }
}
class BCtor extends ACtor implements B {
  b() {
    return this.a().toUpperCase()
  }
  static bar() {
    return 'bar'
  }
}

class DCtor implements D {
  d() {
    return 1
  }
}

class Blank {}

  type ITarget = { foo: string }
  const t: ITarget = {foo: 'bar'}
  const t2 = applyMixinsAsProxy(t, a, b, c, _a)

  t2.c()  // '_a_A'
  t2.a()  // '_a'
  t2.foo  // 'bar'
  // @ts-ignore
  t2.d    // undefined

  type ITarget = { foo: string }
  const t: ITarget = {foo: 'bar'}
  const t2 = applyMixinsAsMerge(t, a, b, c)

  t === t2  // true
  t2.c()    // 'aA'
  t2.a()    // 'a'
  t2.foo    // 'bar'

  const M = applyMixinsAsSubclass(ACtor, Blank, BCtor, DCtor)
  const m = new M()

  M.foo()   // 'foo'
  M.bar()   // 'bar'

  m instanceof M // true
  m instanceof ACtor // true
  m.a()     // 'a'
  m.b()     // 'A'
  m.d()     // 1

  class Target {
    method() {
      return 'value'
    }
  }
  const Derived = applyMixinsAsProto(Target, ACtor, BCtor, DCtor, Blank)
  const m = new Derived()

  Derived === Target // true
  Derived.foo() // 'foo'
  Derived.bar() // 'bar'

  m.a()   // 'a'
  m.b()   // 'A'
  m.d()   // 1

  const n = (n: number) => ({n})
  const m = ({n}: {n: number}) => ({n: 2 * n})
  const k = ({n}: {n: string}) => n.toUpperCase()
  const e = <T extends {}>(e: T): T & {foo: string} => ({...e, foo: 'foo'})
  const i = <T extends {foo: number}>(i: T): T => i

  const nm = applyMixinsAsPipe(n, m)
  const ie = applyMixinsAsPipe(i, e)

  const v1: number = nm(2).n          // 4
  const v2: string = ie({foo: 1}).foo // 'foo'

0. Definition.

   A mixin is a special kind of multiple inheritance.

1. Is it possible to mix classes with automated type inference?

   There're several solutions:

   • A subclass factory
   • Proto merge + constructor invocation + type cast workarounds

2. How to combine OOP and functional mixins?

   Apply different merge strategies for each target type and rest args converters

3. How to check if composition has a given mixin or not?

   Ref Cache / WeakMap

4. What's about mixin factories?

   It's called applyMixins

A mixin is a special kind of multiple inheritance. It's a form of object composition, where component features get mixed into a composite object so that properties of each mixin become properties of the composite object.

In OOP, a mixin is a class that contains methods for use by other classes, and can also be viewed as an interface with implemented methods.

Functional mixins are composable factories which connect together in a pipeline; each function adding some properties or behaviors.

Perhaps these are not perfect definitions, but we'll rely on them.

1. Subclass factory

   type Constructor<T = {}> = new (...args: any[]) => T
   
   function MixFoo<TBase extends Constructor>(Base: TBase) {
     return class extends Base {
       foo() { return 'bar' }
     }
   }

2. Prototype injection

   class Derived {}
   class Mixed {
     foo() { return 'bar' }
   }
   
   Object.getOwnPropertyNames(Mixed.prototype).forEach(name => {
       Object.defineProperty(Derived.prototype, name, Object.getOwnPropertyDescriptor(Mixed.prototype, name));
   })

3. Object assignment

   const foo = {foo: 'foo'}
   const fooMixin = (target) => Object.assign(target, foo)
   const bar = fooMixin({bar: 'bar'})

4. Proxy wrapping

   const mixAsProxy = <P extends IAnyMap, M extends IAnyMap>(target: P, mixin: M): P & M => new Proxy(target, {
     get: (obj, prop: string) => {
       return prop in mixin
         // @ts-ignore
         ? mixin[prop]
         // @ts-ignore
         : obj[prop]
     },
   }) as P & M

5. Functional mixin piping

   const foo = <T>(t: T): T & {foo: string} => ({...t, foo: 'foo'})
   const bar = <T>(t: T): T & {bar: number} => ({...t, bar: 1})
   const foobar = pipe(foo, bar) // smth, that composes fn mixins like `(target) => bar(foo(target))`
   const target = {}
   
   const res = foobar(target)

• medium.com/javascript-scene/functional-mixins-composing-software-ffb66d5e731c
• medium.com/ackee/typescript-function-composition-and-recurrent-types-a9efbc8e7736
• medium.com/@michaelolof/typescript-mix-yet-another-mixin-library-29c7a349b47d
• dev.to/miracleblue/how-2-typescript-get-the-last-item-type-from-a-tuple-of-types-3fh3
• dev.to/ascorbic/creating-a-typed-compose-function-in-typescript-3-351i
• mariusschulz.com/blog/mixin-classes-in-typescript
• justinfagnani.com/2015/12/21/real-mixins-with-javascript-classes/
• github.com/justinfagnani/mixwith.js
• github.com/amercier/es6-mixin
• github.com/Kotarski/ts-functionaltypes
• www.bryntum.com/blog/the-mixin-pattern-in-typescript-all-you-need-to-know/
• stackoverflow.com/questions/533631/what-is-a-mixin-and-why-are-they-useful
• stackoverflow.com/questions/48372465/type-safe-mixin-decorator-in-typescript
• stackoverflow.com/questions/13407036/how-does-interfaces-with-construct-signatures-work

• https://github.com/tannerntannern/ts-mixer
• https://github.com/michaelolof/typescript-mix

MIT